1. Field of the Invention
This invention relates to bulk magnetic components; and more particularly, to a generally three-dimensional high performance bulk metal magnetic component for large electronic devices such as magnetic resonance imaging systems, television and video systems, and electron and ion beam systems.
2. Description of the Prior Art
Certain steel alloys have long been used in magnetic devices in numerous technological applications. The most commonly used of these steels are low-carbon alloys and alloys with up to 3-3.5 weight percent silicon, often referred to as electrical steels and silicon steels, respectively. (As is conventional in the silicon steel art, the content of Si and other elemental additions recited herein is to be understood as a weight percentage unless otherwise specified.) These alloys find widespread use in electric motors, transformers, actuation devices, relays, and the like. Although steels are generally inexpensive, they are often unsuitable for demanding requirements. Among their most significant limitations are their core losses and their magnetostrictions. The low carbon steels are generally the least expensive alloys used for magnetic devices; they are widely available in commerce as unoriented sheet in thicknesses as low as 350 xcexcm (0.014xe2x80x3). However, their core losses are high enough to preclude their use in most applications requiring high efficiency or excitation frequencies greater than line frequency (50-60 Hz). Somewhat lower losses are exhibited by the silicon-containing alloys. They are produced in vast quantities either as non-oriented or oriented sheets in thicknesses as low as 125-175 xcexcm (0.005-0.007xe2x80x3). Oriented sheets have marked crystallographic texture that results in a substantial difference in their magnetic properties for excitation in different directions within the sheet. Oriented sheets are thus most suited for applications wherein flux is predominantly along a defined single direction including transformers and segmented components. Non-oriented materials are best suited to applications wherein the flux direction is not constant during operation, e.g. motor stators.
In addition to steels, other high induction, crystalline materials are known for use in certain magnetic applications, including Fexe2x80x94Sixe2x80x94Al alloys like Sendust, Fexe2x80x94Co alloys, and Fexe2x80x94Ni alloys. In each of these alloy families, small additions of other elements may be added for the sake of metallurgical processing or enhancement of soft magnetic properties.
Magnetic resonance imaging (MRI) has become an important, non-invasive diagnostic tool in modern medicine. An MRI system typically comprises a magnetic field generating device. A number of such field generating devices employ either permanent magnets or electromagnets as a source of magnetomotive force. Frequently the field generating device further comprises a pair of magnetic pole faces defining a gap with the volume to be imaged contained within this gap.
The earliest magnetic pole pieces were made from solid magnetic material such as carbon steel or high purity iron, often known in the art as Armco iron. They have excellent DC properties but very high core loss in the presence of AC fields because of macroscopic eddy currents. Some improvement has been gained by forming a pole piece of laminated conventional steels.
U.S. Pat. No. 4,672,346 teaches a pole face having a solid structure and comprising a plate-like mass formed from a magnetic material such as carbon steel. U.S. Pat. No. 4,818,966 teaches that the magnetic flux generated from the pole pieces of a magnetic field generating device can be concentrated in the gap therebetween by making the peripheral portion of the pole pieces from laminated magnetic plates. U.S. Pat. No. 4,827,235 discloses a pole piece having large saturation magnetization, soft magnetism, and a specific resistance of 20 xcexcxcexa9-cm or more. Soft magnetic materials including permalloy, silicon steel, amorphous magnetic alloy, ferrite, and magnetic composite material are taught for use therein.
U.S. Pat. No. 5,124,651 teaches a nuclear magnetic resonance scanner with a primary field magnet assembly. The assembly includes ferromagnetic upper and lower pole pieces. Each pole piece comprises a plurality of narrow, elongated ferromagnetic rods aligned with their long axes parallel to the polar direction of the respective pole piece. The rods are preferably made of a magnetically permeable alloy such as 1008 steel, soft iron, or the like. The rods are transversely electrically separated from one another by an electrically non-conductive medium, limiting eddy current generation in the plane of the faces of the poles of the field assembly. U.S. Pat. No. 5,283,544, issued Feb. 1, 1994, to Sakurai et al. discloses a magnetic field generating device used for MRI. The devices include a pair of magnetic pole pieces which may comprise a plurality of block-shaped magnetic pole piece members formed by laminating a plurality of non-oriented silicon steel sheets.
Notwithstanding the advances represented by the above disclosures, there remains a need in the art for improved pole pieces. This is so because these pole pieces are essential for improving the imaging capability and quality of MRI systems. Although steel alloys are widely available, they have still been considered unsuitable for use in bulk magnetic components such as the tiles of poleface magnets for advanced magnetic resonance imaging systems (MRI), largely because of their high core losses under AC excitation.
It has also been known in the magnetic materials art that certain advantages might potentially be obtained by using silicon steels with considerably higher silicon content than the typical 3-3.5%. That limit is set by fundamental metallurgical constraints. An alloy with silicon content of greater than about 2.5% is said to have a closed xcex3 loop. That is, upon cooling an alloy with lower than 2.5% Si from high temperature, there is a series of successive allotropic transformations of the alloy from the body-centered cubic (bcc) xcex4 crystallographic phase to the face-centered cubic (fcc) xcex3 phase and finally to the room-temperature bcc xcex1 phase. Instead, at higher Si the alloy remains bcc throughout. This allows a careful interplay of rolling operations and controlled grain growth essential for producing thin-gage, low core loss sheet stock. However, above about 4-4.5% Si there is another difficulty, namely the formation of DO3 and B2 phases which are characterized by superlattice ordering. The presence of the ordered DO3 and B2 phases results in brittleness, precluding normal rolling operations.
It has been recognized that alloys with 6-7% Si have certain attractive electromagnetic characteristics. The increased solute content increases the alloy""s electrical resistivity, tending to improve the eddy current component of core loss. At about 6.5%, the magnetostriction of the alloy is nearly zero, reducing the susceptibility of the component to degradation of its magnetic properties by internally or externally imposed stresses. However, processing difficulties have meant that high silicon iron alloys are still not widely recognized or applied.
Several non-conventional methods have recently been taught for producing sheets of high silicon content Fe-base alloys. First, rapid solidification processing has been used to form directly thin strip material with high Si content. U.S. Pat. No. 4,265,682 to Tsuya et al. discloses a high silicon steel strip consisting of 4-10 weight % of Si and the remainder being substantially Fe and incidental impurities. The strip is produced by rapidly cooling a melt to form a microstructure comprising very fine crystal grains with substantially no ordered lattice. U.S. Pat. Nos. 4,865,657 and 4,990,197, each to Das et al., disclose heat treatment of a rapidly quenched Fexe2x80x94Si containing 6-7 weight % Si to promote and control grain orientation and an order-disorder reaction.
Another method for producing sheets of high silicon content Fe-base alloys is disclosed in U.S. Pat. No. 5,089,061, which teaches subjecting steel strip to siliconization by chemical vapor deposition (CVD) from an atmosphere containing SiCl4 and a subsequent diffusion treatment for diffusing Si uniformly through the steel strip.
Still another method is provided by U.S. Pat. No. 5,489,342, which discloses a method of manufacturing a silicon steel sheet having grains precisely arranged in the Goss orientation, the strip containing 2.5 to 7.0 weight % Si. Goss orientation is a crystallographic texture defined by (110) less than 001 greater than  preferred grain orientation.
Presented is a high performance bulk metal magnetic component having the shape of a polyhedron and being comprised of a plurality of layers of crystalline, ferromagnetic metal strips. Also provided by the present invention is a method for making a high performance, bulk metal magnetic component. The magnetic component is operable at frequencies ranging from about 50 Hz to 20,000 Hz and exhibits improved performance characteristics when compared to conventional silicon-steel magnetic components operated over the same frequency range. More specifically, a magnetic component constructed in accordance with the present invention and excited at an excitation frequency xe2x80x9cfxe2x80x9d to a peak induction level xe2x80x9cBmaxxe2x80x9d may have a core loss at room temperature less than xe2x80x9cLxe2x80x9d wherein L is given by the formula L=0.0135 f (Bmax)1.9+0.000108 f1.6 (Bmax)1.92, the core loss, the excitation frequency and the peak induction level being measured in watts per kilogram, hertz, and teslas, respectively. In an embodiment the magnetic component may have (i) a core-loss of less than or approximately equal to 1 watt-per-kilogram of magnetic metal material when operated at a frequency of approximately 60 Hz and at a flux density of approximately 1.0 Tesla (T); (ii) a core-loss of less than or approximately equal to 20 watts-per-kilogram of magnetic metal material when operated at a frequency of approximately 1000 Hz and at a flux density of approximately 1.0 T, or (iii) a core-loss of less than or approximately equal to 105 watt-per-kilogram of magnetic metal material when operated at a frequency of approximately 20,000 Hz and at a flux density of approximately 0.30 T.
In one embodiment of the present invention, a high performance bulk metal magnetic component comprises a plurality of substantially similarly shaped layers of ferromagnetic metal strips having a high saturation induction and being laminated together by adhesive bonding to form a polyhedrally shaped part.
The present invention also provides methods of constructing a high performance bulk metal magnetic component. In one embodiment of the method, ferromagnetic high saturation induction metal strip material is cut to form a plurality of cut strips having a predetermined length. The cut strips are stacked to form a bar of stacked high saturation induction metal strip material which is impregnated with an epoxy resin and cured. The component is then cut in the requisite shape from the impregnated bar.
In another embodiment of the method, ferromagnetic high saturation induction metal strip material is wound about a mandrel to form a generally rectangular core having generally radiused corners. The core is then impregnated with epoxy resin and cured. The short sides of the rectangular core are then cut to form two magnetic components having a predetermined three-dimensional geometry that is the approximate size and shape of said short sides of said generally rectangular core. The radiused corners are removed from the long sides of said generally rectangular core and the long sides of said generally rectangular core are cut to form a plurality of polyhedrally shaped magnetic components having the predetermined three-dimensional geometry.
Yet another embodiment of the method includes the steps of stamping laminations in the requisite shape from ferromagnetic high saturation induction metal strip feedstock, stacking the laminations to form a three-dimensional shape, applying and activating adhesive means to adhere the laminations to each other forming a component having sufficient mechanical integrity, and finishing the component to remove any excess adhesive and to give it a suitable surface finish and final component dimensions. The method may further comprise an optional annealing step to improve the magnetic properties of the component. These steps may be carried out in a variety of orders and using a variety of techniques including those set forth in more detail hereinbelow.
The present invention is also directed to a bulk high performance metal component constructed in accordance with the above-described methods. In particular, high performance bulk metal magnetic components constructed in accordance with the present invention are especially suited for metal tiles for poleface magnets in high performance MRI systems; television and video systems; and electron and ion beam systems and other devices. The advantages afforded by the present invention include simplified manufacturing, reduced manufacturing time, reduced stresses (e.g., magnetostrictive) encountered during construction of high performance bulk metal components, and optimized performance of the finished magnetic component.